This invention relates to bioremediation of contaminants in the environment. More particularly, this invention relates to crustacean shell and other chitin-containing products as electron donors for environmental bioremediation of contaminants, such as halogenated straight-chain and aromatic hydrocarbons, explosives such as aromatic nitrates, perchlorates, other halogenated organic compounds, and certain metals.
Chlorinated solvents are the most common class of ground water contaminants at hazardous waste sites in the United States. In a list of the top 25 most frequently detected contaminants at such sites, the Agency for Toxic Substances and Disease Registry (ATSDR) found that ten of the top twenty were chlorinated solvents or their degradation products, including two of the top three. National Research Council, Alternatives for Ground Water Cleanup (National Academy Press, Washington, D.C. 1994). In fact, the same survey found that the most common contaminant, trichloroethylene (TCE), is present in more than 40% of the National Priority List sites. Worse yet, remediation of ground water contaminated by these compounds often presents unique obstacles related to their inherent characteristics, including hydrophobicity and high density. Recent advances in the understanding of biodegradation processes involving chlorinated solvents permit remediation of residual contamination source areas in low permeability, saturated or variably saturated soils at a much lower cost than conventional methods.
Metals, perchlorates, explosives, and other contaminants also appear on lists of contaminants frequently detected at hazardous waste sites. These contaminants also present challenges to cleaning up such sites.
Chitin, a condensation polymer of N-acetyl-D-glucosamine, is the structural component of the shells of crustaceans and other arthropods. Chitin is also present in many other organisms, such as fungi and yeasts. Because it is present in so many different types of organisms, chitin is the most plentiful natural polymer next to cellulose.
In arthropods, the intersegmental membranes are flexible chitinous structures. The cuticular sclerites also have the same characteristics just after molting. The bulk of the procuticle is a chitin and protein complex. The innermost layer of the calcified cuticle of crustacea is not calcified, but lies between the calcified procuticle and the epidermis, which has characteristics similar to those of the intersegmental membrane. The sclerites of crustaceans can be interpreted as intersegmental membranes whose procuticle is almost entirely calcified with calcium carbonate and, to a lesser degree, calcium phosphate. Calcium carbonate occurs as micro- or macro-crystals of calcite. The hardening of calcified cuticles is, however, initiated by protein sclerotization prior to deposition of calcium salts. The amount of protein in the calcified cuticle is much lower than in the flexible procuticle or in the sclerotized exocuticle of insects, as a result of the calcification.
In fungi, such as Mortierella vinacea, Mucor rouxii, Phycomyces blakesleeanus, and Cunninghamella elegans, chitin synthetase activity is associated with the cell wall fraction. In other organisms, however, the enzyme was found to be located mainly in the mitochondrial and microsomal fractions. The difficulty of removing chitin synthetase from the cell wall fractions of Mortierella vinacea or Mucor rouxii suggests that the enzyme is bound to the cell wall. In the yeast, Saccharomyces cerevisiae, chitin is found in the primary septum of the cell wall, which occurs at bud scars.
Chitin refers to a polymer of N-acetylglucosamine where a minority of the acetyl groups has been lost, while the term chitosan refers to a deacetylation product obtained from chitin, where most of the acetyl groups have been removed. Experimentally, chitosan can be distinguished from chitin because of its solubility in dilute acetic or formic acid. Chitin is also a product that contains less than 7% nitrogen, while chitosan contains 7% or more nitrogen. The amino groups of chitin and chitosan are exceptionally stable in 50% sodium hydroxide, even at high temperature. Glucosamine occurs as an essential part of the polymer structure. Elemental analysis of chitin samples reveal that they bind water tenaciously. This is consistent with the general picture of chitin structure as a chain of N-acetylglucosamine punctuated by free glucosamine units with considerable amounts of trapped water as part of the molecule.
Chitin and chitosan are both biodegradable and non-toxic, and they have binding properties such that they function as excellent flocculants for clarifying liquids, help heal wounds, can be fabricated into strong permeable films, and function as drug-delivery gels for topical application of a variety of medicaments. It has also been determined that chitin may be used as an electron donor in bioremediation of contaminants in the environment.
Chitin and chitosan are generally refined from the waste products of the crab and shrimp industries, but can also be produced from the processing wastes of shellfish, krill, clams, oysters, and fungi. The crustacean shells are treated with a caustic wash (i.e., strong base, such as sodium hydroxide) to remove protein and other contaminants attached to the shells. The resulting intermediate, which is primarily composed of chitin, mineral complexes (primarily calcium carbonates), and water, is then washed with water to remove residual base. Next, the deproteinized intermediate is treated with strong acid, such as hydrochloric acid, for dissolving and removing the minerals. After the minerals are washed away, the product is dried and may be ground and screened for size. Riccardo A. A. Muzzarelli, Chitin (Pergamon 1977). The resulting refined, “pure” chitin typically contains greater than 94.4% chitin, 1.5% ash, and 3% caustic solubles on a dry weight basis. The biggest deficiency of such a chitin product for environmental bioremediation is the high cost of the chitin in quantities sufficient for use in the field.
While prior art methods of using chitin as an electron donor for bioremediation applications are known and are generally suitable for their limited purposes, they possess certain inherent deficiencies, such as high cost and low efficiency, that detract from their overall utility in environmental bioremediation.
In view of the foregoing, it will be appreciated that providing high-efficiency methods for carrying out bioremediation of hazardous waste sites using low-cost, chitin-containing materials would be a significant advancement in the art.